1. Field of the Invention
The present invention relates to honeycomb structure bodies for use in an electric heating catalyst device capable of purifying exhaust gas emitted from an internal combustion engine mounted to motor vehicles, industrial plants, etc.
2. Description of the Related Art
In general, a catalyst device is mounted to an exhaust gas pipe of an exhaust gas purifying system mounted to a motor vehicle. For example, exhaust gas is emitted from an internal combustion engine and is passing through the exhaust gas pipe in the exhaust gas purifying system. The exhaust gas is purified by the catalyst device mounted on the exhaust gas pipe. The purified exhaust gas is then discharged to the outside of the internal combustion engine of the motor vehicle. The catalyst device uses a honeycomb structure body. The honeycomb structure body supports catalyst such as platinum (Pt), palladium (Pd), rhodium (Rh), etc. It is necessary to heat the catalyst supported in the honeycomb structure body to approximately 400° C. in order to adequately activate the catalyst. In order to increase a temperature of the honeycomb structure body in the catalyst device up to a necessary temperature and to activate the catalyst supported in the honeycomb structure body, a conventional technique provides an electric heating catalyst (EHC) device. The EHC device has a honeycomb structure body, an outer skin section and a pair of electrodes. The honeycomb structure body is equipped with a honeycomb body. The pair of the electrodes is formed on the outer circumferential surface of the honeycomb body. When electrical power is supplied to the electrodes, current flows in the honeycomb body, and heat energy is generated and the catalyst is heated to an optimum temperature to activate the catalyst. For example, a prior patent document, Japanese patent laid open publication No. JP H04-280086, discloses a honeycomb structure body having a conventional structure in which a pair of electrodes is formed on an outer surface of a honeycomb body, and an electrode terminal is formed on the corresponding electrode.
However, the honeycomb structure body disclosed in Japanese patent laid open publication No. JP H04-280086 has a drawback to easily transmit heat energy of exhaust gas passing through the inside of the honeycomb body to the electrode terminals because the electrode terminals have a large heat capacity. This decreases a temperature of the electrodes directly under the electrode terminals and the sections near to the electrode terminals. As a result, a temperature difference occurs along an axial direction (or a longitudinal direction) of the honeycomb body through which exhaust gas is passing. In general, when the honeycomb structure body is frequently used in a cooling/heating cycle, thermal stress is easily caused by the temperature difference in the section of the honeycomb body directly under the electrode terminals and the sections of the honeycomb body close to the electrodes. As a result, cracks are generated in the electrodes and the inside of the honeycomb body by the thermal stress.